Vacuum fuse

ABSTRACT

A vacuum fuse is provided which exhibits current ratings from 150 to 300 amps in a 15-kv system. The fuse includes a pair of arcing electrodes in the form of parallel, confronting, spaced apart disks supported on the inner ends of aligned contact rods. The confronting surfaces of the electrodes define an arc gap and are provided with a plurality of arcuate grooves. A fusible element projects from areas which are recessed relative to the confronting disk surfaces to bridge the arc gap and it and the arcing electrodes are electrically connected to the contact rods. The fusible element is flattened at the mid-point of the arc gap and a hole is drilled in the flattened section. The outer housing of the fuse includes a pair of cylindrical insulators each one connected to a contact rod by an end cap and positioned on an axial side of the arcing electrodes. The outer housing is enlarged in the area of the arcing electrodes by two bell-shaped metallic sections which have a smaller cross section portion attached to one of the insulators and flare outwardly to enlarged diameter portions engaging each other. An interior metallic, cylindrical shield is connected in the enlarged area of the housing in registry with the arc gap. The electrical and structural connections are brazed with a vacuum being pulled on the fuse interior during the brazing operation. All structural elements are arranged with the longitudinal axis of the final assembly as the reference point.

United States Patent [1 1 [111 3,913,047

Arthur et al. Oct. 14, 1975 VACUUM FUSE [75] Inventors: Michael E. Arthur, South [57] ABSTRACT Milwaukee; Harvey W. Mikulecky, Racine, both of Wis.; John W. Ranheim, San Jose, Calif.

[73] Assignee: McGraw-Edison Company, Elgin,

Ill.

[22] Filed: Apr. 25, 1974 [21] Appl. No.: 464,103

[52] US. Cl 337/17; 200/144 B; 337/28; 337/34 [51] Int. Cl. HOIH 61/00 [58] Field of Search 317/62, 66, 77; 200/144 B;

[56] References Cited UNITED STATES PATENTS 2,906,841 9/1959 Jennings 317/62 3,190,986 6/1965 Jennings 337/252 X 3,244,839 4/1966 Albright 337/159 X 3,328,632 6/1967 Robinson.... 317/62 X 3,348,007 10/1967 Urani 337/252 X 3,411,038 11/1968 Lee ZOO/144 B 3,522,570 8/1970 Wanaselja 337/32 X 3,705,281 12/1972 Loehring..... ZOO/144 B 3,813,577 5/1974 Kawiecke 337/32 X Primary Examiner-.1. D. Miller Assistant ExaminerFred E. Bell A vacuum fuse is provided which exhibits current ratings from 150 to 300 amps in a l5-kv system. The fuse includes a pair of arcing electrodes in the form of parallel, confronting, spaced apart disks supported on the inner ends of aligned contact rods. The confronting surfaces of the electrodes define an arc gap and are provided with a plurality of arcuate grooves. A fusible element projects from areas which are recessed relative to the confronting disk surfaces to bridge the arc gap and it and the arcing electrodes are electrically connected to the contact rods. The fusible element is flattened at the mid-point of the arc gap and a hole is drilled in the flattened section. The outer housing of the fuse includes a pair of cylindrical insulators each one connected to a contact rod by an end cap and positioned on an axial side of the arcing electrodes. The outer housing is enlarged in the area of the arcing electrodes by two bell-shaped metallic sections which have a smaller cross section portion attached to one of the insulators and flare outwardly to enlarged diameter portions engaging each other. An interior metallic, cylindrical shield is connected in the enlarged area of the housing in registry with the arc gap. The electrical and structural connections are brazed with a vacuum being pulled on the fuse interior during the brazing operation. All structural elements are arranged with the longitudinal axis of the final assembly as the reference point.

17 Claims, 2 Drawing Figures VACUUM FUSE BACKGROUND OF THE INVENTION This invention relates to vacuum fuses.

Electrical installations such as underground distribution systems have grown in popularity and also in size and complexity. With this growth has come an increased demand for sectionalizing equipment. Current limiting fuses, oil fuse cutouts, fused load interrupters and the like have been used but such equipment suffers from various drawbacks such as cost and limited current carrying and interrupting capacity. This invention is concerned with these problems and proposes a vacuum fuse construction which is well suited to use in electrical installations of this type.

SUMMARY OF THE INVENTION In accordance with this invention, a pair of spaced, confronting arcing surfaces are enclosed within a sealed housing. The space between the arcing surfaces is bridged by a fusible element which is electrically connected to spaced contact rods which rods are also electrically connected to the arcing surfaces. The sealed housing is made up of insulating members surrounding a portion of and having a sealed connection to each of the contact rods, the insulating members are connected by an enlarged central section to define an enlarged area in which the arcing surfaces are located. The interior space defined by the housing is evacuated through an opening into the housing interior which opening is later sealed to maintain the vacuum.

In accordance with more specific aspects of this invention, the fusible element extends from areas which are recessed relative to the arcing surfaces to insure that the arc which is drawn when the fusible element melts will transfer to the arcing surfaces and also to maintain desired melting characteristics; the arcing surfaces are defined by electrically conductive disks which provide heat sink masses for the arc; a metallic ring is provided around the interior of the housing in alignment with the space between the arcing sufaces to provide a shield against which the arcing products are expelled; arcuate grooves are provided in the confronting arcing surfaces to influence outward travel of the arc; and/or the fusible element, at a location generally in the center of the space between the arcing surfaces, is provided with a reduced cross section to determine the i r characteristics and the point of arc initiation.

Additionally, it is preferred that the component elements of the vacuum fuse be arranged relative to the longitudinal axis of the fuse. Specifically, the contact rods are arranged end-to-end on the axis, the arcing surfaces, or disks, are centered on the axis, the fusible element and, where provided, the recesses are on the axis, and the insulating members and the central body portion are concentrically arranged on the axis. This permits the entire assembly to be held in a jig for ease of assembly and it can then be held assembled as brazing compound is applied to the various joints and connections and the entire assembly is processed in a braz ing oven. The added advantage is that while being brazed evacuation of the interior of the housing can be initiated and continued after brazing is complete to insure pulling the desired vacuum. Drawing the vacuum is achieved by providing an opening into the interior of the housing through an exposed end of one of the contact rods, this is later sealed (while still at an elevated temperature) to insure maintaining the proper vacuum.

De-gasification of the housing interior is optimized by assembling the fusible element into axial holes in the 5 contact rods which axial holes register with radial holes through which mechanical staking of the fusible element and/or insertion of the brazing compound can be accomplished; the openings, primarily the radial openings, expose all worked areas to the vacuum drawing and minimizes the occurrence of trapped gases.

Other objects and advantages will be pointed out in, or be apparent from, the specification and claims, as will obvious modifications of the embodiment shown in the drawings, in which:

FIG. 1 is an axial section through a vacuum fuse; and FIG. 2 is a section taken generally along line 22 in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The vacuum fuse depicted in the drawings includes a housing 1 made up of high voltage insulators 2 and 3, end caps 4 and 6, and bell-shaped housing members 7 and 8. The high voltage insulators are made of suitable electrical insulating material such as high alumina, forsterite, other ceramic materials, glass, or glassceramic. The end caps and bell-shaped housing members may be made of suitable metallic material such as copper-nickel, iron-nickel, cobalt or stainless steel, which materials should also be compatible with the insulator material so that the insulators are joinable to the end caps.

This housing structure defines an interior area 9. The fuse elements housed within this area include elongated contact rods 1 1 and 12 and arcing electrodes 13 and 14 supported on the inner ends of the contact rods. The arcing electrodes are generally disk-shaped and are arranged parallel to each other to define confronting arcing surfaces 16 and 17 between which gap 18 is defined. Fuse element 19 is electrically connected to the contact rods 11 and 12, as are disks l3 and 14, and the fuse element extends between the rods bridging gap 18. Fuse element 19 is preferably in the form of an OFI-IC (oxygen free high conductivity) copper wire of appropriate diameter and is provided with a flattened section 21 which is located between arcing surfaces 16 and 17. Hole 22 is drilled in the flattened section and is located at the center of the gap 18 relative to arcing surfaces 16 and 17. This combination of flattened section and hole 22 provides a reduced cross section area in the gap to permit precise definition of the melting i t characteristics of the fuse and, furthermore, insures initiation of arcing in the center of gap 18. Other wire configurations are possible to provide the-desired minimum cross section area in the center of gap 18. The diameter of the fuse element is selected to provide the particular desired ampere rating. For example, a number 11 copper wire will produce a 200 ampere rating.

Contact rods 11 and 12 and disks 13 and 14 can be made from OFI-IC or other de-oxidized copper.

Preferably, the fuse element is supported in and extends from areas which are recessed with respect to arcing surfaces 16 and 17. In the preferred embodiment these recesses are provided by drilled bores 23 and 24 in the adjacent ends of contact rods 11 and 12. The reasons for the desirability of supporting the fuse element in these recessed areas are that, when the fuse is called upon to operate the fuse element must melt,

by recessing the point of support for the fuse element the heat sink effect of disks l3 and 14 (which after the arc is drawn is a desirable feature) is minimized resulting in maximum heat retention in the fusible element and thus a shorter melting time; secondly, with the recess, as the fuse element melts back to points approximately flush with the arcing surfaces 16 and 17, the arc drawn will transfer immediately to the arcing surfaces for proper arc extinction as will be evident from the discussion hereinafter; and support of the fuse element in the recesses eliminates the possibility of residual fuse element portions protruding into the gap which would make the fuse more prone to restrike or reignite at less than fault conditions.

Upon melting, the fuse element burns back to a point flush with arcing surfaces 16 and 17 at which point the arc will transfer to the arcing surfaces and, in accordance with well known phenomena, will move outwardly on the arcing surfaces toward the periphery of disks 13 and 14. Disks l3 and 14 are beveled adjacent their periphery at areas 26 and 27, this increases the length of the arc as it approaches the periphery of the disks and enhances extinction of the arc when current zero condition occurs. 7

As will be described more completely hereinafter, a vacuum condition is created in area 9 to achieve the desired fuse operation. During interruption, products of arcing are expelled outwardly from between disks 13 and 14. To protect the joint between housing members 7 and 8 as well as the housing members themselves from damage from these products of the arc, shield 28 in the form of a cylindrical ring is connected to the interior surface of members 7 and 8. This shield can be of an OFHC copper or stainless steel and products of the are are caused to impinge on that shield as opposed to the housing elements to assist in maintaining the integrity of the vacuum condition within the fuse.

To complete the description of the general structure of the fuse, it will be noted that contact rods 11 and 12 project outwardly from end caps 4 and 6. These projections of the contact rods provide the media through which electrical connection of the fuse can be made in the particular circuit to be rpotected. Various forms of conventional mounting of the fuse can be utilized and arenot shown, for example a clip-style mounting or the like can be used.

Looking now more specifically to the preferred structure of the fuse as illustrated in the drawing, the overall fuse is elongated and has a longitudinal axis which coincides generally with the center or axis of contact rods 11 and 12. The various structural elements of the fuse are formed and assembled using that longitudinal axis as a reference point. The contact rods are arranged end-to-end on that axis, the disks l3 and 14 and arcing surfaces 16 and 17, which the disks cooperate in defining, are centered on that axis, and fuse element 19 is also located on the axis as are bores 23 and 24. With reference to the outer housing, insulators 2 and 3 are generally cylindrical and surround a portion of contact rods 11 and 12 and are concentrically arranged on the longitudinal axis. The insulators 2 and 3 are located one on either axial side of the combinationdisks l3 and 14 with the ends 3land 32thereof, which face away from the disks being connected to the contact rodsby' end caps 4 and 6. The other axial ends 33 and 34 of the insulators are connected to housing members 7 and 8.

At this point it will be noted that housing members 7 and 8 are generally bell-shaped having ends 36 and 38 of reduced diameter corresponding generally to the diameter of cylinders 2 and 3 and flare outwardly from those reduced diameters to enlarged diameter portions I 39 and 41. The enlarged diameter portions 39 and 41 are generally aligned with disks l3 and 14 and the gap 18 defined between the arcing surfaces to thereby define an enlarged mid-section in which the arc stream travels. This enlarged mid-section provides a desirable long arc path along which the arc runs during extinction, permits relatively large mass disks l3 and 14 to insure proper functioning of the fuse while the neckeddown configuration provided by the bell-shaped members and insulators 2 and 3 maintains a minimum interior volume for more effective vacuum conditions. This also requires less insulator material which is a relatively high cost item.

An opening from the exterior into interior area 9 is provided through which the vacuum can be drawn. Preferably, this opening 42 is provided in one of the contact rods 12. Pinch-off tube 43 is provided in opening 42 and after the desired vacuum condition has been achieved, the tube is pinched off permanently. A solder plug 44 is provided on the end of the pinch-off tube for protection.

With the just described structural arrangement of the elements, it is then possible to assembly all of the elements in their final orientation in a suitable jig. Being jig assembled, the elements can then be held together for a brazing operation. Prior to the brazing operation, brazing compound is provided at the connections of the end caps with the contact rods and the insulator members, between the insulator members and tne bellshaped members 7 and 8, and at the overlap joints 46 and 48 between the bell-shaped members. The jig assembled parts with the applied brazing compound is then introduced into a suitable brazing oven in which a brazing temperature is maintained, e.g. about 840 C. The use of brazed connections provides a very reliable seal at the various connection points. Also, it is possible then to evacuate the interior 9 during the brazing cycle and subsequent cooling cycles or, if desired, it can be accomplished in a system separate from the brazing furnace but with appropriate bake-out and pumping cycles. The advantage of drawing the vacuum at an elevated temperature in this manner is well known and doing it during the baking cycle does have the advantage of eliminating one of the processing steps. Rel ztively high vacuums of from 10" to 10 torr can achieved in the fuse and, as importantly, because of the positively sealed structure of the fuse and its structural integrity, the integrity of the vacuum can be maintained over a relatively long service life.

In this respect, it should be noted at this point that the metallic-ceramic housing construction (insulators 2 and 3 and metallic members 7 and 8) provides a housing which will withstand the high brazing or bake-out temperatures and also insures structural integrity in service.

A further refinement is found in the manner in which fuse element 19 is supported. The fuse element extends into drilled, axial openings 49 and 51 in contact rods 11 and 12. Radial openings 52 and 53 are also provided in the contact rods and intersect the axial openings. With this arrangement the fuse element is inserted in the axial openings and the radial openings permit a staking operation to be performed on the fuse elements to mechanically hold them in place. Moreover, brazing compound can be inserted through openings 52 and 53 to further insure positive mechanical and electrical connections. Having provided the radial and axial openings, complete de-gasification of the interior of the fuse is insured even at the brazed connections of the fuse element.

Grooves 54 can be provided in disks 13 and 14, the grooves taking the form of openings through the thickness of the disks and being generally arcuate in configuration. The arcuate grooves spiral outwardly from the central area of the disks and influence arc travel from the point of the initial arc outwardly toward the periphery of the disks.

With this fuse construction, vacuum fuses having current ratings from 150 to 300 amps for -kv systems can be provided, higher ampere and voltage ratings can be achieved as can fuses having even lower ratings. The vacuum fuse is mass producible and insures vacuum integrity over a relatively long service life. It should be remembered that the fuse may be in service for a number of years before it is called upon to interrupt and thus the vacuum must be maintained for long periods and the described construction provides that vacuum integrity. An effective compromise is achieved between minimum interior area to insure an effective vacuum while still maintaining adequate space for are extinction as well as relatively long arc runner surfaces (disks 13 and 14). This also permits relatively large disks to be used to provide heat sinks to absorb the energy which is created during arcing and for arc extinction.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

We claim:

1. A vacuum fuse comprising, in combination,

means defining first and second spaced, generally parallel, confronting arcing surfaces arranged in general registry with each other,

fusible element means connected to and bridging the space between said arcing surfaces,

first and second electrically conductive contact rods electrically connected to said fusible element means and said arcing surfaces and projecting oppositely relative to the space between said arcing surfaces,

a housing including an electrical insulating member surrounding a portion of and connected to each of said contact rods and each insulating member spaced from said arcing surfaces along a respective one of said contact rods,

a central housing section enlarged relative to and extending between said insulating members enclosing the space between said arcing surfaces,

and means providing a seal connection between said housing section and said insulating members and between said insulating members and contact rods so that the interior of said housing in which said arcing surfaces are arranged is sealed,

and means defining an opening into said housing interior through which the atmosphere in said housing interior can be evacuated and which can be sealed 4 after a vacuum has been achieved in said housing interior.

2. The vacuum fuse of claim 1 including means defining opposed recesses in said confronting surfaces,

and wherein said fusible element means is connected in said recesses.

3. The vacuum fuse of claim 1 wherein said means defining said arcing surfaces includes first and second spaced, electrically conductive disks.

4. The vacuum fuse of claim 1 including a generally cylindrical shaped shield connected to the interior of said central housing section spaced from and in registry around said space between said arcing surfaces.

5. The vacuum fuse of claim 1 including means defining a plurality of generally arcuate grooves in each of said arcing surfaces.

6. The vacuum fuse of claim 1 wherein said fusible element means is an elongated member having a reduced cross section area therein located centrally relative to said space between said arcing surfaces.

7. A vacuum fuse having a longitudinal axis and comprising, in combination,

first and second contact rods arranged end-to-end on said axis but spaced from each other,

means connected to each of said contact rods and defining spaced, confronting arcing surfaces, said means including first and second disks centered on said axis and projecting radially from said contact rods,

an elongated fusible element located generally on said axis and electrically connected to said contact rods and bridging the space between said arcing surfaces, first and second electrical insulating members surrounding portions of said first and second contact rods, respectively, and spaced along said contact rods from said space between arcing surfaces,

means providing a sealed connection of said first and second insulating members to. said first and second contact rods,

the diameters of said first and second insulating members being less than the diameters of said disks,

first and second generally bell-shaped members each having a reduced diameter portion and flaring outwardly to an enlarged diameter portion, means providing a sealed connection of said reduced diameter portions one to each of said first and second insulating members and the enlarged diameter portion of one bell-shaped member to the enlarged diameter portion of the other, said enlarged diameter portions aligned radially relative to said disks and the space between said arcing electrodes,

whereby said disks and said arcing surfaces are located within a sealed area,

and means defining an opening into said sealed area through which said sealed area can be evacuated and then sealed to maintain a vacuum in said sealed area.

8. The vacuum fuse of claim 7 including means defining opposed recesses in said confronting surfaces,

and wherein said fusible element means is connected in said recesses.

9. The vacuum fuse of claim 8 wherein said opening into said sealed area is provided in one of said contact rods.

10. The vacuum fuse of claim 9 wherein said arcing surfaces are the confronting surfaces of said disks,

including means defining a plurality of arcuate openings through said disks extending generally outward from the central area of said surfaces.

11. The vacuum fuse of claim 10 including axial openings in each of said contact rods opening into each of said recesses,

including radial openings in each of said contact rods intersecting said axial openings and exposed to said sealed area,

and wherein said fusible element extends into both of said axial openings and over said radial openings.

12. The vacuum fuse of claim 9 including axial openings in each of said contact rods opening into each of said recesses,

including radial openings in each of said contact rods intersecting said axial openings and exposed to said sealed area,

and wherein said fusible element extends into both of said axial openings and over said radial openings.

13. The vacuum fuse of claim 11 including means defining brazed joints comprising, in combination,

first and second elongated contact rods arranged end-to-end on said axis but spaced from each other,

first and second circular disks electrically connected one to each of the adjacent ends of said contact rods and centered on said axis, said disks relatively parallel and spaced from each other defining a space therebetween,

means defining opposed areas on said axis recessed relative to the confronting surfaces of said disks,

a fusible element electrically connected to said contact rods and bridging the space between said confronting disk surfaces.

means defining a reduced cross section area in said fusible element located generally in the center of said space,

first and second cylindrical insulating members each having opposite axial ends and each surrounding a portion of said first and second contact rods respectively,

an end cap having a sealed connection between each respective contact rod and the axial end of each of said insulating members which faces away from said disks,

first and second bell-shaped members having reduced diameter portions connected one to each of the axial ends of said insulating members facing toward said disks, said bell-shaped members flaring generally outward from said reduced diameter portions to enlarged diameter portions having a sealed engagement with each other whereby said end caps, insulating members and bell-shaped members define a sealed interior area,

said disks and the space therebetween being located in the area defined by said enlarged diameter portions of said bell-shaped members,

means defining an opening in one of said contact rods extending from the exterior into said interior sealed area through which said sealed interior area can be evacuated,

and means for sealing said opening to maintain a vacuum in said sealed area.

16., The vacuum fuse of claim 15 wherein said arcing surfaces are the confronting surfaces of said disks,

including means defining a plurality of arcuate openings through said disks extending generally outward from the central area of said surfaces.

17. The vacuum fuse of claim 15 including means defining brazed joints between said insulating members and said contact rods,

between said bell-shaped members and said insulating members and one bell-shaped member to another,

and between said fusible element and said contact rods. 

1. A vacuum fuse comprising, in combination, means defining first and second spaced, generally parallel, confronting arcing surfaces arranged in general registry with each other, fusible element means connected to and bridging the space between said arcing surfaces, first and second electrically conductive contact rods electrically connected to said fusible element means and said arcing surfaces and projecting oppositely relative to the space between said arcing surfaces, a housing including an electrical insulating member surrounding a portion of and connected to each of said contact rods and each insulating member spaced from said arcing surfaces along a respective one of said contact rods, a central housing section enlarged relative to and extending between said insulating members enclosing the space between said arcing surfaces, and means providing a seal connection between said housing section and said insulating members and between said insulating members and contact rods so that the interior of said housing in which said arcing surfaces are arranged is sealed, and means defining an opening into said housing interior through which the atmosphere in said housing interior can be evacuated and which can be sealed after a vacuum has been achieved in said housing interior.
 2. The vacuum fuse of claim 1 including means defining opposed recesses in said confronting surfaces, and wherein said fusible element means is connected in said recesses.
 3. The vacuum fuse of claim 1 wherein said means defining said arcing surfaces includes first and second spaced, electrically conductive disks.
 4. The vacuum fuse of claim 1 including a generally cylindrical shaped shield connected to the interior of said central housing section spaced from and in registry around said space between said arcing surfaces.
 5. The vacuum fuse of claim 1 including meaNs defining a plurality of generally arcuate grooves in each of said arcing surfaces.
 6. The vacuum fuse of claim 1 wherein said fusible element means is an elongated member having a reduced cross section area therein located centrally relative to said space between said arcing surfaces.
 7. A vacuum fuse having a longitudinal axis and comprising, in combination, first and second contact rods arranged end-to-end on said axis but spaced from each other, means connected to each of said contact rods and defining spaced, confronting arcing surfaces, said means including first and second disks centered on said axis and projecting radially from said contact rods, an elongated fusible element located generally on said axis and electrically connected to said contact rods and bridging the space between said arcing surfaces, first and second electrical insulating members surrounding portions of said first and second contact rods, respectively, and spaced along said contact rods from said space between arcing surfaces, means providing a sealed connection of said first and second insulating members to said first and second contact rods, the diameters of said first and second insulating members being less than the diameters of said disks, first and second generally bell-shaped members each having a reduced diameter portion and flaring outwardly to an enlarged diameter portion, means providing a sealed connection of said reduced diameter portions one to each of said first and second insulating members and the enlarged diameter portion of one bell-shaped member to the enlarged diameter portion of the other, said enlarged diameter portions aligned radially relative to said disks and the space between said arcing electrodes, whereby said disks and said arcing surfaces are located within a sealed area, and means defining an opening into said sealed area through which said sealed area can be evacuated and then sealed to maintain a vacuum in said sealed area.
 8. The vacuum fuse of claim 7 including means defining opposed recesses in said confronting surfaces, and wherein said fusible element means is connected in said recesses.
 9. The vacuum fuse of claim 8 wherein said opening into said sealed area is provided in one of said contact rods.
 10. The vacuum fuse of claim 9 wherein said arcing surfaces are the confronting surfaces of said disks, including means defining a plurality of arcuate openings through said disks extending generally outward from the central area of said surfaces.
 11. The vacuum fuse of claim 10 including axial openings in each of said contact rods opening into each of said recesses, including radial openings in each of said contact rods intersecting said axial openings and exposed to said sealed area, and wherein said fusible element extends into both of said axial openings and over said radial openings.
 12. The vacuum fuse of claim 9 including axial openings in each of said contact rods opening into each of said recesses, including radial openings in each of said contact rods intersecting said axial openings and exposed to said sealed area, and wherein said fusible element extends into both of said axial openings and over said radial openings.
 13. The vacuum fuse of claim 11 including means defining brazed joints between said insulating members and said contact rods, between said bell-shaped members and said insulating members and one bell-shaped member to another, and between said fusible element and said contact rods.
 14. The vacuum fuse of claim 13 including a cylindrical metallic shield member connected to the interior of said bell-shaped members generally in radial alignment with said space between said arcing surfaces.
 15. A vacuum fuse having a longitudinal axis and comprising, in combination, first and second elongated contact rods arranged end-to-end on said axis but spaced from each other, firsT and second circular disks electrically connected one to each of the adjacent ends of said contact rods and centered on said axis, said disks relatively parallel and spaced from each other defining a space therebetween, means defining opposed areas on said axis recessed relative to the confronting surfaces of said disks, a fusible element electrically connected to said contact rods and bridging the space between said confronting disk surfaces. means defining a reduced cross section area in said fusible element located generally in the center of said space, first and second cylindrical insulating members each having opposite axial ends and each surrounding a portion of said first and second contact rods respectively, an end cap having a sealed connection between each respective contact rod and the axial end of each of said insulating members which faces away from said disks, first and second bell-shaped members having reduced diameter portions connected one to each of the axial ends of said insulating members facing toward said disks, said bell-shaped members flaring generally outward from said reduced diameter portions to enlarged diameter portions having a sealed engagement with each other whereby said end caps, insulating members and bell-shaped members define a sealed interior area, said disks and the space therebetween being located in the area defined by said enlarged diameter portions of said bell-shaped members, means defining an opening in one of said contact rods extending from the exterior into said interior sealed area through which said sealed interior area can be evacuated, and means for sealing said opening to maintain a vacuum in said sealed area.
 16. The vacuum fuse of claim 15 wherein said arcing surfaces are the confronting surfaces of said disks, including means defining a plurality of arcuate openings through said disks extending generally outward from the central area of said surfaces.
 17. The vacuum fuse of claim 15 including means defining brazed joints between said insulating members and said contact rods, between said bell-shaped members and said insulating members and one bell-shaped member to another, and between said fusible element and said contact rods. 